In the description of the background that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present disclosure.
The term “nanostructure” material is used by those familiar with the art to designate materials including nanoparticles such as C60 fullerenes, fullerene-type concentric graphitic particles, metal, compound semiconductors such as CdSe, InP; nanowires/nanorods such as Si, Ge, SiOx, GeOx, or nanotubes composed of either single or multiple elements such as carbon, BxNy, CxByN2, MoS2, and WS2. One of the common features of nanostructure materials is their basic building blocks. A single nanoparticle or a carbon nanotube has a dimension that is less than 500 nm at least in one direction. These types of materials have been shown to exhibit certain properties that have raised interest in a variety of applications and processes.
U.S. Pat. Nos. 6,280,697 and 6,422,450 to Zhou et al. (both entitled “Nanotube-Based High Energy Material and Method”), the disclosures of which are incorporated herein by reference, in their entirety, disclose the fabrication of carbon-based nanotube materials and their use as a battery electrode material.
U.S. Pat. No. 6,630,772 (Ser. No. 09/296,572 entitled “Device Comprising Carbon Nanotube Field Emitter Structure and Process for Forming Device”) the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube-based electron emitter structure.
U.S. Pat. No. 6,630,772 to Bower et al. (entitled “Device Comprising Thin Film Carbon Nanotube Electron Field Emitter Structure”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon-nanotube field emitter structure having a high emitted current density.
U.S. Pat. No. 6,277,318 to Bower et al. (entitled “Method for Fabrication of Patterned Carbon Nanotube Films”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a method of fabricating adherent, patterned carbon nanotube films onto a substrate.
U.S. Pat. No. 6,334,939 (entitled “Nanostructure-Based High Energy Material and Method”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a nanostructure alloy with alkali metal as one of the components. Such materials are described as being useful in certain battery applications.
U.S. Pat. No. 6,553,096 to Zhou et al. (entitled “X-Ray Generating Mechanism Using Electron Field Emission Cathode”), the disclosure of which is incorporated herein by reference, in its entirety, discloses an X-ray generating device incorporating a nanostructure-containing material.
U.S. Published Patent Application No. US 2002/0140336 (entitled “Coated Electrode With Enhanced Electron Emission And Ignition Characteristics”), the disclosure of which is incorporated herein by reference, in its entirety, discloses an electrode including a first electrode material, an adhesion-promoter, and a carbon nanotube-containing material disposed on at least a portion of the adhesion promoting layer, as well as associated devices incorporating such an electrode.
U.S. Patent Publication No. US 2002/0193040 (entitled “Method of Making Nanotube-Based Material With Enhanced Field Emission”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a technique for introducing a foreign species into the nanotube-based material in order to improve the properties thereof.
U.S. Patent Publication No. US 2002/0094064 (entitled “Large-Area Individually Addressable Multi-Beam X-Ray System and Method of Forming Same”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a structure to generate x-rays having a plurality of stationary and individually electrically addressable field emissive electron sources, such as carbon nanotubes.
U.S. Patent Publication No. US 2003/0180472 (entitled “Method for Assembling Nano-objects”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a technique for the self assembly of a macroscopic structure with preformed nano-objects, which may be processed to render a desired aspect ratio and/or chemical functionality.
As evidenced by the above, nanostructure materials, especially those such as carbon nanotubes and other nano-objects having a large aspect ratio (i.e.—a length which is substantially larger than its diameter) possess promising properties that make them attractive for a variety of applications, such as lighting elements, field emission devices such as flat panel displays, gas discharge tubes for over voltage protection, x-ray generating devices, small conduction wires, sensors, actuators and high resolution probes such as those used in scanning microscopes.
The effective incorporation of such materials into such devices has been hindered by difficulties encountered in the processing of such materials. For instance, nanostructured materials can be formed by techniques such as laser ablation, and arc discharge methods, solution synthesis, chemical etching, molecular beam epitaxy, chemical vapor deposition, laser ablation, etc. However, processing techniques to assemble these nanostructure materials have posed certain difficulties.
Post-formation methods such as screen printing and spraying have been utilized to deposit pre-formed nano-objects such as carbon nanotubes on a substrate. However, such techniques pose certain drawbacks. For instance, screen printing requires the use of binder materials as well as an activation step. Spraying can be inefficient and is not practical for large-scale fabrication. Moreover, these techniques typically result in randomly distributed nanostructure materials on the substrate.
Carbon nanotubes have been grown directly upon substrates by use of chemical vapor deposition (CVD) techniques. See, e.g.—J. Hafner et al., Nature, Vol. 398, pg. 761, 1999 and U.S. Pat. Nos. 6,457,350 and 6,401,526. One potential application of this technique is the formation of conducting wires made from nanostructure materials, such as carbon nanotubes and electrical circuitry. The CVD process can be used to form the conducting wires which are attached to electrodes at specific locations using CVD techniques to form the conducting wires. However, such techniques require relatively high temperatures (e.g.—600–1,000° C.) as well as reactive environments, and the use of catalysts in order to effectively grow the nanotubes. The requirement for such harsh environmental conditions severely limits the types of substrate materials which can be utilized. In addition, the CVD technique often results in mutli-wall carbon nanotubes. These mutli-wall carbon nanotubes generally do not have the same level of structural perfection and thus have inferior electronic emission properties when compared with single-walled carbon nanotubes. Also, direct growth of nanotubes onto the substrate by such techniques makes it difficult to control the length, orientation and number of the nanotubes deposited thereby.
Other techniques have involved efforts to precisely control the deposition of individual or small groups of nano-objects, such as carbon nanotubes, onto a substrate, such as sharp tips or projections. See, e.g.—Dai, Nature, Vol. 384, pgs. 147–150 (1996); and R. Stevens et al., Appl. Phys. Lett., Vol. 77, pg. 3453. However, such techniques are painstaking and time-consuming, and do not lend themselves to efficient large-scale production, or batch processing. For example, U.S. Pat. No. 6,528,785 describes a process by which plate-like electrodes are placed in an electrophoretic solution and nanotubes are deposited on at least one of the electrodes. The electrode(s) is withdrawn from the solution and nanotubes deposited thereon are transferred to a sharp tip in a further processing step. The nanotube(s) is then “fusion welded” to the tip by yet another processing step which may include the deposition of a coating material over at least the portion of the nanotube attached the sharp tip. The process is slow and lacks of control of the orientation. The tips formed usually comprise one carbon nanotube (CNT) per tip. The interfacial bonding between the tip and CNT tends to be weak. It is difficult to fabricate many tips at one time. This process is undesirably complicated and tedious, and thus is impractical for commercial scale production.
Another consideration in the art is that in the construction of electrical devices using nanostructured materials, it is often necessary to have materials with the same properties, such as their electronic properties. This has not been achieved. For example, single wall carbon nanotubes materials synthesized by the laser ablation methods contain materials that are both metallic and semiconducting by nature. Currently, there is no effective method to separate the nanotubes based on their properties. For instance, separation of metallic and semiconducting nanotubes is necessary for many device applications.